The presently disclosed subject matter relates to a pulse photometer, and also to a method for evaluating the reliability of a calculated value of the blood light absorber concentration.
A pulse photometer is an apparatus which calculates the blood light absorber concentration of the subject. Specifically, the living tissue of the subject is irradiated with light beams at a plurality of wavelengths which have different ratios of the blood light absorbances depending on the blood light absorber concentration. The intensities of the light beams at the wavelengths transmitted through or reflected from the living tissue are detected. The intensities at the wavelengths are changed in accordance with the pulsation of the blood in the subject. Therefore, temporal changes of the intensities at the wavelengths due to the pulsation are acquired in the form of a pulse wave signal. The amplitudes of pulse wave signals with respect to waveforms correspond to light attenuation variations with respect to the waveforms, respectively. The blood light absorber concentration is calculated based on a ratio of light attenuation variations with respect to waveforms (for example, see Japanese Patent No. 4,196,209).
As an example of the blood light absorber concentration, known is the arterial oxygen saturation (hereinafter, referred to as the SaO2) which is used as an index of blood oxygenation. In order to obtain the value of the SaO2, an invasive measurement must be performed. Therefore, the transcutaneous arterial oxygen saturation (hereinafter, referred to as the SpO2) which can be non-invasively calculated is widely used as the index. The SpO2 is calculated by a pulse oximeter which is an example of a pulse photometer.
Techniques for evaluating the reliability of a calculated value which is obtained as described above are available. In the technique disclosed in Japanese Patent No. 4,865,737, for examples, a calculated value is compared with a data value which is previously acquired in a predetermined measurement environment, and the reliability of the calculated value is evaluated in accordance with the difference between the values.